Redundant drive train for pylon mounted rotors

ABSTRACT

A system includes an inboard tiltrotor subsystem and an outboard tiltrotor subsystem. The inboard tiltrotor subsystem includes an inboard pylon, an inboard tiltrotor, and a single and non-redundant drivetrain. The outboard tiltrotor subsystem includes an outboard pylon that is coupled to a wing and an outboard tiltrotor. The outboard tiltrotor has a range of motion and is coupled to the wing via the outboard pylon, such that the outboard tiltrotor is aft of the wing. The outboard tiltrotor subsystem further includes a redundant drivetrain (which has a plurality of motors and a plurality of motor controllers) that drives one or more blades and the one or more blades.

CROSS REFERENCE TO OTHER APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.17/066,944 entitled REDUNDANT DRIVE TRAIN FOR PYLON MOUNTED ROTORS filedOct. 9, 2020 which is incorporated herein by reference for all purposes,which claims priority to U.S. Provisional Patent Application No.62/912,872 entitled FIXED WING AIRCRAFT WITH TILT ROTORS filed Oct. 9,2019 which is incorporated herein by reference for all purposes.

BACKGROUND OF THE INVENTION

New types of aircraft are being developed which are capable of takingoff and landing in dense urban areas, opening up new transportationpathways and bypassing gridlock on the roads. For example, Kitty HawkCorporation is developing a new electric vertical takeoff and landing(eVTOL) tiltrotor which can take off and land in a footprint of roughly30 ft.×30 ft. An early prototype has been manufactured and tested andfurther improvements to the vehicle's performance (e.g., improvingsafety and/or reducing mass) would be desirable.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments of the invention are disclosed in the followingdetailed description and the accompanying drawings.

FIG. 1A is a perspective view diagram illustrating an embodiment of aforward swept, fixed wing vehicle with tilt rotors.

FIG. 1B is a top view diagram illustrating an embodiment of a forwardswept, fixed wing vehicle with tilt rotors.

FIG. 2A is a diagram illustrating a bottom view of an embodiment ofboundary layer thicknesses with the motors off.

FIG. 2B is a diagram illustrating a bottom view of an embodiment ofboundary layer thicknesses with motors on.

FIG. 3A is a diagram illustrating an example of a tilt wingconfiguration with corresponding lift vector, thrust vector, and drag.

FIG. 3B is a diagram illustrating an example of a fixed wingconfiguration with a leading edge mounted tilt rotor and correspondinglift vector, thrust vector, and drag.

FIG. 3C is a diagram illustrating an embodiment of a fixed wingconfiguration with a trailing edge mounted tilt rotor and correspondinglift vector, thrust vector, and drag.

FIG. 4 is a diagram illustrating an embodiment of airflow produced whentrailing edge mounted tilt rotors on a main wing are off.

FIG. 5 is a diagram illustrating an embodiment of a forward swept andtapered wing and a straight wing for comparison.

FIG. 6A is a diagram illustrating an embodiment of a takeoff tilt changefrom hover position to cruise position.

FIG. 6B is a diagram illustrating an embodiment of a landing tilt changefrom cruise position to hover position.

FIG. 7 is a diagram illustrating an embodiment of a velocity tiltdiagram.

FIG. 8A is a front view diagram of an embodiment of a redundantdrivetrain system with a belt-driven shaft.

FIG. 8B is a side view diagram of an embodiment of a redundantdrivetrain system with a belt-driven shaft.

FIG. 8C is a top view diagram of an embodiment of a redundant drivetrainsystem with a belt-driven shaft.

FIG. 8D is a perspective diagram of an embodiment of a redundantdrivetrain system with a belt-driven shaft.

FIG. 9 is a diagram illustrating an embodiment of a pylon-mountedtiltrotor in a hovering position.

FIG. 10A is a front view diagram of an embodiment of a redundantdrivetrain system configured to fit into a pylon with a gear-drivenshaft.

FIG. 10B is a side view diagram of an embodiment of a redundantdrivetrain system configured to fit into a pylon with a gear-drivenshaft.

FIG. 10C is a top view diagram of an embodiment of a redundantdrivetrain system configured to fit into a pylon with a gear-drivenshaft.

FIG. 10D is a perspective view diagram of an embodiment of a redundantdrivetrain system configured to fit into a pylon with a gear-drivenshaft.

FIG. 11A is a front view diagram of embodiments of a redundantdrivetrain system configured to fit into a pylon with coaxially-arrangeddrivetrains.

FIG. 11B is a top view diagram of embodiments of a redundant drivetrainsystem configured to fit into a pylon with coaxially-arrangeddrivetrains.

FIG. 11C is a perspective view diagram of embodiments of a redundantdrivetrain system configured to fit into a pylon with coaxially-arrangeddrivetrains.

DETAILED DESCRIPTION

The invention can be implemented in numerous ways, including as aprocess; an apparatus; a system; a composition of matter; a computerprogram product embodied on a computer readable storage medium; and/or aprocessor, such as a processor configured to execute instructions storedon and/or provided by a memory coupled to the processor. In thisspecification, these implementations, or any other form that theinvention may take, may be referred to as techniques. In general, theorder of the steps of disclosed processes may be altered within thescope of the invention. Unless stated otherwise, a component such as aprocessor or a memory described as being configured to perform a taskmay be implemented as a general component that is temporarily configuredto perform the task at a given time or a specific component that ismanufactured to perform the task. As used herein, the term ‘processor’refers to one or more devices, circuits, and/or processing coresconfigured to process data, such as computer program instructions.

A detailed description of one or more embodiments of the invention isprovided below along with accompanying figures that illustrate theprinciples of the invention. The invention is described in connectionwith such embodiments, but the invention is not limited to anyembodiment. The scope of the invention is limited only by the claims andthe invention encompasses numerous alternatives, modifications andequivalents. Numerous specific details are set forth in the followingdescription in order to provide a thorough understanding of theinvention. These details are provided for the purpose of example and theinvention may be practiced according to the claims without some or allof these specific details. For the purpose of clarity, technicalmaterial that is known in the technical fields related to the inventionhas not been described in detail so that the invention is notunnecessarily obscured.

Various embodiments of a redundant drivetrain system are describedherein. In a redundant drive train, there are multiple drivetrains,including multiple motors and multiple motor controllers. As will bedescribed in more detail below, redundant drivetrain systems may reducethe total drivetrain weight and in some cases permit the remainingoperational drivetrain(s) to output at least some thrust from thepropeller. In some embodiments, the system includes a pylon, where thepylon is coupled to a wing (e.g., a main wing or a canard) and atiltrotor, where the tiltrotor has a range of motion, the tiltrotor iscoupled to the wing via the pylon, such that the tiltrotor is aft of thewing, and the tiltrotor includes a redundant drivetrain, including aplurality of motors and a plurality of motor controllers, that drivesone or more blades included in the tiltrotor.

First, an exemplary tiltrotor vehicle is described where initialprototypes used non-redundant drivetrains for each propeller or rotor.As will be described in more detail below, the uniqueness of theexemplary vehicle applied a unique set of constraints on the redundantdrivetrain systems that could be used therein. It is noted that theredundant drivetrain system described herein may be used in othervehicles beyond the exemplary tiltrotor vehicle.

FIG. 1A is a perspective view diagram illustrating an embodiment of aforward swept, fixed wing vehicle with tilt rotors. FIG. 1B is a topview diagram illustrating an embodiment of a forward swept, fixed wingvehicle with tilt rotors. In the example shown, the main wing (100 a and100 b) is a fixed wing which is attached to the fuselage (102 a and 102b) in a fixed manner or position. The main wing is not, in other words,a tilt wing which is capable of rotating. The main wing (100 a and 100b) is also forward swept (e.g., relative to the pitch axis). Forexample, the forward-sweep angle may be on the order of ° sweep between14° and 16° for aircraft embodiments with a canard (as shown here) or ashigh as 35° for aircraft embodiments without a canard.

In this example, the main wing (100 a and 100 b) has six rotors (104 aand 104 b) which are attached to the trailing edge of the main wing.Rotors or propellers in this configuration are sometimes referred to aspusher propellers (e.g., because the propellers are behind the wing and“push” the vehicle forward, at least when they are in their forwardflight position). Forward flight mode is sometimes referred to herein ascruise mode. For clarity, these rotors on the main wing are sometimesreferred to as the main wing rotors (e.g., to differentiate them fromthe rotors which are attached to the canard). Naturally, the number ofrotors shown here is merely exemplary and is not intended to belimiting.

In addition to the six main wing rotors, there are two rotors (106 a and106 b) which are attached to the canard (108 a and 108 b). These rotorsare sometimes referred to as the canard rotors. The canard is thinnerthan the main wing, so unlike the main wing rotors, the canard rotorsare attached to the distal ends of the canard as opposed to the trailingedge of the canard.

All of the rotors in this example are tilt rotors, meaning that they arecapable of tilting or otherwise rotating between two positions. In thedrawings shown here, the rotors are in a cruise (e.g., forward flight,backward facing, etc.) position. In this position, the rotors arerotating about the (e.g., substantially) longitudinal axes of rotationso that they provide (substantially) backward thrust. When the rotorsare in this position, the lift to keep the tiltrotor vehicle airbornecomes from the airflow over the main wing (100 a and 100 b) and thecanard (108 a and 108 b). In this particular example, the rotationalrange of a tilt rotor ranges has a minimum angular position ofapproximately 0°-5° and a maximum angular position of approximately90°-95°. This range is design and/or implementation specific.

The rotors can also be tilted down to be in a hover (e.g., verticaltakeoff and landing, downward facing, etc.) position (not shown). Inthis second position, the rotors are rotating about (e.g.,substantially) vertical axes of rotation so that they provide(substantially) downward thrust. In this configuration, the lift to keepthe tiltrotor vehicle airborne comes from the downward airflow of therotors.

Generally speaking, the tilt rotors, when oriented to output thrustsubstantially downward, permit the aircraft to perform vertical takeoffand landings (VTOL). This mode or configuration (e.g., with respect tothe manner in which the aircraft as a whole is flown and/or with respectto the position of the tilt rotors specifically) is sometimes referredto as hovering. The ability to perform vertical takeoffs and landingspermits the aircraft to take off and land in areas where there are noairports and/or runways. Once airborne, the tilt rotors (if desired)change position to output thrust (substantially) backwards instead ofdownwards. This permits the aircraft to fly in a manner that is moreefficient for forward flight; this mode or configuration is sometimesreferred to as cruising.

A canard is useful because it can stall first (e.g., before the mainwing), creating a lot of pitching moments and not much loss of lift atstall whereas a main wing stall loses a lot of lift per change inpitching moment (e.g., causing the entire aircraft to drop or fall).Stalls are thus potentially more benign with a canard compared towithout a canard. The canard stall behavior is particularly beneficialin combination with a swept forward wing, as the stall of the main wingcan create an adverse pitching moment if at the wing root and can createlarge and dangerous rolling moments if at the wing tip. Furthermore, acanard can create lift at low airspeeds and increase CL_(max) (i.e.,maximum lift coefficient) and provides a strut to hold or otherwiseattach the canard motors to.

In some embodiments, the pylons (110 a and 110 b) which are used toattach the rotors to the canard and/or main wing include some hingeand/or rotating mechanism so that the tilt rotors can rotate between thetwo positions shown. Any appropriate hinge mechanism may be used. Forexample, with ultralight aircraft, there are very stringent weightrequirements and so a lightweight solution may be desirable.Alternatively, a fixed-tilt solution may also be used to meet verystringent weight requirements.

In some embodiments, the aircraft is designed so that the main wing (100a and 100 b) and canard (108 a and 108 b) are able to provide sufficientlift to perform a glider-like landing if needed during an emergency. Forexample, some ultralight standards or specifications require the abilityto land safely if one or more rotors fail and the ability to perform aglider-like landing would satisfy that requirement. One benefit to usinga fixed wing for the main wing (e.g., as opposed to a tilt wing) is thatthere is no danger of the wing being stuck in the wrong position (e.g.,a hover position) where a glider-like landing is not possible because ofthe wing position which is unsuitable for a glider-like landing.

Another benefit to a fixed wing with trailing edge mounted tilt rotorsis stall behavior (or lack thereof) during a transition from hoverposition to cruise position or vice versa. With a tilt wing, duringtransition, the tilt wing's angle of attack changes which makes stallingan increased risk. A fixed wing with trailing edge mounted tilt rotorsdoes not change the wing angle of attack (e.g., even if rotors areturned off/on or the tilt rotors are shifted). Also, this configurationboth adds dynamic pressure and circulation over the main wing, whichsubstantially improves the behavior during a transition (e.g., fromhover position to cruise position or vice versa). In other words, thetransition can be performed faster and/or more efficiently with a fixedwing with trailing edge mounted tilt rotors compared to a tilt wing (asan example).

Another benefit associated with fixed wing vehicle with tilt rotors(e.g., as opposed to a tilt wing) is that a smaller mass fraction isused for the tilt actuator(s). That is, multiple actuators for multipletilt rotors (still) comprise a smaller mass fraction than a single,heavy actuator for a tilt wing. There are also fewer points of failurewith tilt rotors since there are multiple actuators as opposed to asingle (and heavy) actuator for the tilt wing. Another benefit is that afixed wing makes the transition (e.g., between a cruising mode orposition and a hovering mode or position) more stable and/or fastercompared to a tilt wing design.

In some embodiments, the rotors are variable pitch propellers which havedifferent blade pitches when the rotors are in the hovering positionversus the cruising position. For example, different (ranges of) bladepitches may enable more efficient operation or flight when in the cruiseposition versus the hovering position. When the rotors are in a cruiseposition, putting the blade pitches into “cruising pitch” (e.g., on theorder of 26°) enables low frontal area which is good for cruising (e.g.,lower drag). When the rotors are in a hovering position, putting theblade pitches into a “hovering pitch” (e.g., on the order of 6°) enableshigh disc area which is good for hovering. To put it another way, oneblade pitch may be well suited for cruising mode but not for hoveringmode and vice versa. The use of variable pitch propellers enables better(e.g., overall) efficiency, resulting in less power consumption and/orincreased flight range.

The following figures illustrate various benefits associated with theexemplary aircraft shown in FIGS. 1A and 1B.

FIG. 2A is a diagram illustrating a bottom view of an embodiment ofboundary layer thicknesses with the motors off. In this example, laminarrun lines 200 a, 202 a, and 204 a illustrate laminar runs at variousregions of the main wing. In this example, it is assumed that theaircraft is cruising (e.g., flying in a substantially forwarddirection). As in FIGS. 1A and 1B, the main wing rotors (206) areattached to the trailing edge of the main wing (208) in this embodiment.The next figure shows the boundary layer thicknesses with the rotorsturned on.

FIG. 2B is a diagram illustrating a bottom view of an embodiment ofboundary layer thicknesses with motors on. In this example, the motorsare on and the rotors have an exit airflow velocity of 30 m/s. With themotors on, a low pressure region is created towards the aft of the wingwhich increases the laminar run on the main wing. See, for example,laminar run lines 200 b, 202 b, and 204 b which correspond to laminarrun lines 200 a, 202 a, and 204 a from FIG. 2A. A comparison of the twosets illustrates that the laminar runs have increased for the first twolocations (i.e., at 200 a/200 b and 202 a/202 b). The last location(i.e., 204 a/204 b) has only a slightly longer laminar run length due tointerference from the canard rotors (210).

The drag from the main wing rotors (more specifically, the drag from thepylons which are used to attach the main wing rotors to the main wing)is hidden in the wake of the airflow coming off the main wing. See, forexample FIG. 2A which more clearly shows that the pylons (220) areconnected or otherwise attached behind most of the extent of laminar run(222). With the embodiment shown here, the pylons also get to keep someof the boundary layer thickness from the main wing, which means thepylons have lower drag per surface area. This improves the drag comparedto some other alternate designs or configurations. The following figuresdescribe this in more detail.

FIG. 3A is a diagram illustrating an example of a tilt wingconfiguration with corresponding lift vector, thrust vector, and drag.In this example, a fixed rotor (300) is attached to a tilt wing (302) ata fixed position or angle. This is one alternate arrangement to theaircraft embodiment(s) described above. To direct the airflow producedby the fixed rotor (300) either backwards or downwards, the tilt wing(302) is rotated. As shown here, with this configuration, there is drag(304) at the trailing edge of the tilt wing, which is undesirable.

The lift (306) and thrust (308) for this configuration are also shownhere, where the tilt wing is shown in the middle of a transition (e.g.,between a cruising position and a hovering position). As shown here, thelift (306) and thrust (308) are substantially orthogonal to each other,which is inefficient. In other words, a tilt wing is inefficient duringits transition.

FIG. 3B is a diagram illustrating an example of a fixed wingconfiguration with a leading edge mounted tilt rotor and correspondinglift vector, thrust vector, and drag. In this example, a tilt rotor(320) is attached to the leading edge of a fixed wing (322). This isanother alternate arrangement to the aircraft embodiment(s) describedabove. The corresponding drag (324) and thrust (326) for thisarrangement are also shown. There is no useful lift produced with thisconfiguration and therefore no lift vector is shown here.

FIG. 3C is a diagram illustrating an embodiment of a fixed wingconfiguration with a trailing edge mounted tilt rotor and correspondinglift vector, thrust vector, and drag. In this example, the tilt rotor(340) is attached to the trailing edge of the fixed wing (342). In thisconfiguration, the drag due to the trailing edge mounted tilt rotor(e.g., mostly due to its pylon, not shown) is hidden in the wake of theairflow coming off the main wing. As such, there is no drag (at leastdue to the tilt rotor (340)).

The position of the trailing edge mounted tilt rotor (340) relative tothe fixed wing (342) also sucks air (344) over the fixed wing, afterwhich the air turns or bends through the rotor and downwards. This flowturning over the wing generates a relatively large induced lift (346)which is shown here. The thrust vector (348) due to the rotors is alsoshown here. It is noted that the induced lift (346) and thrust (348) aresubstantially in the same direction (i.e., both are pointingsubstantially upwards) which is a more efficient arrangement, includingduring a transition. In other words, using a fixed wing with trailingedge mounted tilt rotors produces less drag and improved efficiencyduring a transition (e.g., due to the lift and thrust vectors which nowpoint in substantially the same direction) compared to other rotor andwing arrangements. Note for example, drag 304 and drag 324 in FIG. 3Aand FIG. 3B, respectively, and the orthogonal positions of lift 306 andthrust 308 in FIG. 3A.

The following figure illustrates an embodiment of flow turning in moredetail.

FIG. 4 is a diagram illustrating an embodiment of airflow produced whentrailing edge mounted tilt rotors on a main wing are off. In thisexample, a tiltrotor (400) is shown but with the main wing rotors turnedoff for comparison purposes. With the rotors off, the airflow in (402)and the airflow out (404) are moving in substantially the samedirection. That is, the airflow does not turn (e.g., downwards) as itpasses through the rotors.

Tiltrotor 420 shows the same vehicle as tiltrotor 400 except the rotorsare turned on. In this example, the airflow in (422) and the airflow out(424) have noticeable different directions and there is noticeableturning or bending of the airflow as it passes through the rotors of theexemplary tiltrotor shown. As described above, this induces a noticeablelift, which is desirable because less power is consumed and/or the rangeof the tiltrotor increases.

In this example, the main wing rotors (426) are in the hoveringposition. As shown here, these rotors are slightly pitched or otherwiseangled (e.g., with the tops of the main wing rotors pointing slightlyforward and the bottoms pointing slightly backward). In this diagram,the amount of tilting is shown as θ_(pitch) (428) and in someembodiments is on the order of 90° of rotational range or movement(e.g., ˜3° up from horizontal when in a cruise position (e.g., forminimum drag) and ˜93° degrees down from horizontal when in a hoverposition which produces a rotational range of ˜96°). Although thisangling or pitching of the rotors is not absolutely necessary for flowturning to occur, in some embodiments the main wing rotors are angled orotherwise pitched to some degree in order to increase or otherwiseoptimize the amount of flow turning. In some embodiments, the canardrotors are similarly pitched. It is noted that tiltrotor 420 is shown ina nose up position and therefore the vertical axis (e.g., relative tothe tiltrotor) is not perpendicular to the ground and/or frame ofreference.

In some embodiments, the rotors (e.g., the main wing rotors and/orcanard rotors) are rolled or otherwise angled slightly outward, awayfrom the fuselage, when the rotors are in hovering position. In someembodiments, this roll (e.g., outward) is on the order of 10° forgreater yaw authority.

In some embodiments, the main wing is tapered (e.g., the wing narrowsgoing outward towards the tip) in addition to being forward swept. Thefollowing figures describe various wing and/or tail embodiments.

FIG. 5 is a diagram illustrating an embodiment of a forward swept andtapered wing and a straight wing for comparison. In the example shown,wing 500 is a straight wing with no tapering (e.g., the wing is the samewidth from the center to the tip of the wing). Exemplary rotors (502)are shown at the trailing edge of the straight wing (500).

The center of thrust (504), indicated by a dashed and dotted line, isdictated by the placement or arrangement of the rotors and runs throughthe centers of the main wing rotors (502). For simplicity, the canardrotors are ignored in this example. The center of lift is based on theshape of the wing. For a rectangular wing such as wing 500, the centerof lift (506), indicated by a solid line, runs down the center of thewing. Calculation of the aerodynamic center is more complicated (e.g.,the aerodynamic center depends upon the cross section of the wing, etc.)and aerodynamic center 508, indicated by a dashed line, is exemplaryand/or typical for this type of wing.

As shown here, the straight wing (500) and its corresponding arrangementof main wing rotors (502) produce a center of thrust (504) which isrelatively far from both the center of lift (506) as well as theaerodynamic center. This separation is undesirable. More specifically,when the main wing rotors (502) are in hover position, if the center ofthrust (504) is far from the center of lift (506), then the transition(e.g., in the context of the movement of the aircraft as a whole, suchas switching from flying substantially upwards to substantially forwardsor vice versa) would create very large moments and could overturn thevehicle or prevent acceleration or stability and/or require a massiveand/or non-optimal propulsion system. In cruise, if the center of thrust(504) is far from the center of lift (506), it is not as important(e.g., since the thrust moments are both smaller and more easilybalanced by aerodynamic moments), but it is still undesirable.

In contrast, the forward swept and tapered wing (520) and itscorresponding arrangement of rotors (522) along the trailing edgeproduce a center of thrust (524), center of lift (526), and aerodynamiccenter (528) which are closer to each other. For example, the forwardsweep of the wing brings the rotors forward to varying degrees. Thiscauses the center of thrust to move forward (e.g., towards the leadingedge and towards the other centers). The tapering of the wings preventsthe aerodynamic center and center of lift from creeping forward too much(and more importantly, away from the center of thrust) as a result ofthe forward sweep. For example, with a forward swept wing with notapering (not shown), the center of thrust would move forwardapproximately the same amount as the aerodynamic center and center oflift and would result in more separation between the three centers thanis shown here with wing 520.

Some other benefits to a forward swept and tapered wing include betterpilot visibility, and a better fuselage junction location with the mainwing (e.g., so that the main wing spar can pass behind the pilot seat,not through the pilot). Furthermore, the taper reduces wing moments andputs the center of the thrust of the motors closer to the wingattachment to the fuselage, as referenced about the direction of flight,so there are less moments carried from wing to fuselage, a shorter tailboom (e.g., which reduces the weight of the aircraft), and improvedpitch stability.

The following figures describe exemplary tilt transitions of the rotorsbetween cruise position and hover position.

FIG. 6A is a diagram illustrating an embodiment of a takeoff tilt changefrom hover position to cruise position. In some embodiments, theexemplary tiltrotor performs this transition soon after taking off(e.g., substantially vertically). It is noted that this tilt transitionis optional and the aircraft may fly entirely with the rotors in thehovering position (albeit with less than optimal performance). Forexample, this could be done if there is risk in the tilting action, andit would be better to take the action at a higher altitude.

Tiltrotor 600 shows the exemplary aircraft after it has performed avertical takeoff. In this state shown here, the main wing rotors andcanard rotors are in hover position (e.g., rotating about asubstantially vertical axis of rotation so that the rotors generatesubstantially downward thrust).

The tiltrotor then transitions from an entirely upward direction ofmovement to a direction of movement with at least some forward motionwith the rotors remaining in the hover position until the tiltrotorreaches some desired altitude at which to begin the transition (602). Inother words, the vehicle transitions first, and then changes the tilt ofthe rotors. In one example, the altitude at which the tiltrotor beginsthe rotor tilt change from hover position to cruise position is analtitude which is sufficiently high enough for there to be recovery timein case something goes wrong during the transition. Switching the rotorsbetween hover position and cruise position is a riskier time where thelikelihood of something going wrong (e.g., a rotor failing, a rotorgetting stuck, etc.) is higher. Although the tiltrotor may have systemsand/or techniques in place for recovery (e.g., compensating for a rotorbeing out by having the remaining rotors output more thrust, deploy aparachute, etc.), these systems and/or techniques take time (i.e.,sufficient altitude) to work.

From position 602, the tiltrotor flies substantially forward and movesthe tilt rotors from a hover position (e.g., where thrust is outputsubstantially downward) to a cruise position. Once in the cruiseposition 604, the rotors rotate about a substantially longitudinal axisso that they output backward thrust.

FIG. 6B is a diagram illustrating an embodiment of a landing tilt changefrom cruise position to hover position. For example, the exemplarytiltrotor may perform this transition before landing vertically. As withthe previous transition, this transition is optional. For example, theexemplary tiltrotor can keep the tilt rotors in cruise position andperform a glider-like landing as opposed to a vertical landing ifdesired.

Tiltrotor 610 shows the rotors in a cruise position. While flying in asubstantially forward direction, the tilt rotors are moved from thecruise position shown at 610 to the hover position shown at 612. Withthe tilt rotors in the hover position (612), the tiltrotor descends withsome forward movement (at least in this example) so as to keep power uselow(er) and retain better options in the case of a failure of a motor orother component (e.g., the tiltrotor can power up the rotors and pullout of the landing process or path) to position 614 until it finallylands on the ground.

FIG. 7 is a diagram illustrating an embodiment of a velocity tiltdiagram. In the diagram shown, the x-axis shows the forward speed of theaircraft and the y-axis shows the tilt (e.g., position or angle of thetilt wing or tilt rotors) which ranges from a (e.g., minimal) cruiseposition (700) to a (e.g., maximal) hover position (702).

The first operating envelope (704), shown with a solid border and filledwith a grid pattern, is associated with a tilt wing aircraft. See, forexample, tiltrotor 400 in FIG. 4 and tilt wing 302 and fixed rotor 300in FIG. 3A. The second operating envelope (706), shown with a dashedborder and gray fill, is associated with an (e.g., comparable) aircraftwith a forward swept and fixed wing with trailing edge mounted tiltrotors. See, for example, the embodiments described above.

In the diagram shown here, the tilt rotor operating envelope (706) is asuperset of the tilt wing operating envelope (704) which indicates thatthe former aircraft configuration is safer and/or more airworthy thanthe latter and is also able to fly both faster and slower at comparabletilt positions. With a fixed wing, the wing is already (and/or always)pointed in the direction of (forward) travel. When the tilt rotors areat or near the (e.g., maximal) hover position (702), the vehicle can flypretty much all the way up to the stall speed (e.g., V₂) without havingto tilt the motors up to cruise position. Note, for example, that thetilt rotor operating envelope (706) can stay at the (e.g., maximal)hover position (702) all the way up to V₂. This greatly increases theoperating regime of the tilt rotor operating envelope (706) compared tothe tilt wing operating envelope (704). Note for example, all of thegray area above the tilt wing operating envelope (704).

Another effect which can contribute to the expanded operating envelopefor the tilt rotor configuration at or near hover position includes flowturning (see, e.g., FIG. 4 ). The flow turning over the main winginduces some extra lift. In some embodiments, this flow turning and itsresulting lift are amplified or optimized by tilting the main wingrotors at a slight backward angle from directly down when in a normalhover (e.g., at minimal tilt position 700).

In contrast, a tilt wing presents a large frontal area when the tiltwing is tilted up in (e.g., maximal) hover position (702). As a result,tilt wings are unable to fly forward at any kind of decent speed untilat or near the full (e.g., minimal) cruise position (700) or nearly so.

Early prototypes of the tiltrotor vehicle were directed to proof ofconcept and developing an airworthy vehicle. To that end, each propellerhad a single drivetrain for simplicity. The following figures describevarious embodiments of redundant drivetrain systems that may be usedwith the exemplary tiltrotor vehicle or other vehicles.

FIG. 8A is a front view diagram of an embodiment of a redundantdrivetrain system with a belt-driven shaft. FIGS. 8B-8D show a sideview, a top view, and a perspective view, respectively, of the samedrivetrain embodiment. In the example shown, the redundant drivetrainsystem is used to drive the blades of the tiltrotor where the tiltrotoris coupled to a wing or canard via a fixed pylon, as described above.The rotor disc (800 a-800 d) is sufficiently small enough to fit withinthe pylon subsystem.

A first drivetrain (802 a-802 d), including a first motor and a firstmotor controller, and a second drivetrain (804 a-804 d), including asecond motor and a second motor controller, drive a shaft (806 a-806 d)which holds the propeller. In this example, the two drivetrains drivethe shaft via a belt (808 a-808 d).

With the vehicle shown in FIGS. 1A-1B, there would be a total for 16drivetrains with the redundant drivetrain shown in FIGS. 8A-8D. If oneof the 16 drivetrains were to fail, the other drivetrains would need tosupport 1.15 times their normal load in hover (e.g., assuming a simplecontrol scheme the opposing unit would not be able to contribute much).Let us suppose one of the outermost motors or controllers fails on theleft side of the main wing. In this case, to maintain balance, in thesimplest of control schemes, the opposing rotor (i.e., the outermostrotor on the right side of the main wing), would only be able to provideas much thrust as the remaining part of the one on the left wing. Ifthere are no redundant systems, the one on the left would provide nothrust and neither would the one on the right. With redundancy, bothwould be able to provide about half the regular thrust.

In contrast, with the previous powertrain architecture with a total of 8powertrain units, if one of the power trains were to fail, the otherdrivetrains would need to support 1.33 times their normal load in hover.Redundant drivetrains per propeller reduce the power output required ofeach propeller in the event of a failure. This, in turn, permits thetotal mass of the (redundant) drivetrain system to be reduced, for amass savings of ⅙ of the total drivetrain mass (e.g., since total massesare driven by failure cases). Some of this savings might be offset byincrease in For context, the stack height of the motor in this exampleis half that of the original drivetrain architecture (e.g., with onedrivetrain per propeller).

One attractive feature of a belt-driven redundant drivetrain system (asshown here) is that drivetrain failures are more benign since no rotorshave to stop completely. This may make belt-driven drivetrains anacceptable choice for some applications even though belt drives are notas efficient as direct drives and the side-by-side arrangement of themotors results in a wider pylon housing which in turn increases drag.The efficiency loss (e.g., ˜5% loss) and increase in drag (e.g.,˜10%-15%) may be acceptable tradeoffs to have a partially drivenpropeller.

The uniqueness of the exemplary tiltrotor vehicle described above meansthat there are some specific design constraints that a redundantdrivetrain system used in such a vehicle would need to satisfy. Forexample, the redundant drivetrain system must fit inside thepylon-mounted tiltrotors. The following figure shows a more detailedexample of the pylon-mounted tiltrotors.

FIG. 9 is a diagram illustrating an embodiment of a pylon-mountedtiltrotor in a hovering position. In the example shown, a forward, pylonportion (900) fits around the wing or canard (902). Aft of the pylon isthe tiltrotor portion (904), shown herein in the hovering position. Thetiltrotor in this example is an earlier version with a single motor(906) which drives the shaft (908) and in turn the propeller blades(910). It would be desirable for a redundant drivetrain system to fit(substantially) in the existing footprint shown here, since increasingthe size of the forward, pylon portion (900) or the aft, tiltrotorportion (904) would increase the surface area which in turn wouldincrease drag. For context, the diameter of the tiltrotor (904) is 160mm and the length from the forward surface (912) to the plane of theblades (910) is 600 mm.

In addition to the belt-drive embodiment shown in FIGS. 8A-8D, aredundant drivetrain may be implemented in variety of embodiments. Thefollowing figures show some alternatives.

FIG. 10A is a front view diagram of an embodiment of a redundantdrivetrain system configured to fit into a pylon with a gear-drivenshaft. FIGS. 10B-10D show a side view, a top view, and a perspectiveview, respectively, of the same drivetrain embodiment. As in the exampleof FIGS. 8A-8D, the motors are reduced in size and power output byreducing the stack height by half (e.g., compared to the originalarchitecture with a single powertrain per propeller). Like the previousexample, the components shown reside within a cylindrical space (1000a-1000 d), such as a tapered, cylindrically-shaped tiltrotor that is aftof a fixed pylon portion that couples the tiltrotor to a wing or canard.A first drivetrain (1002 a-1002 d) and a second drivetrain (1004 a-1004d) turn a gear-driven shaft (1006 a-1006 d) which in turn drives thepropeller. In some applications, the relatively low loss in agear-driven system (e.g., ˜2% or less) may make such an embodimentattractive, even if some types of failures may prohibit the remainingand functioning drivetrains from driving the shaft via gears.

FIG. 11A is a front view diagram of embodiments of a redundantdrivetrain system configured to fit into a pylon with coaxially-arrangeddrivetrains. FIGS. 11B-11C show a top view and a perspective view,respectively, of the same drivetrain embodiments.

In the examples shown here, the diagrams on the left (1100 a-1100 c)show a singular, non-redundant drivetrain for comparison purposes.

The center diagrams (1102 a-1102 c) show a redundant drivetrainembodiment with equivalent power where the motor diameter is maintainedbut the stack height is halved.

The diagrams on the right (1104 a-1104 c) show a setup where diameter isreduced and stack height is increased a bit compared to the originaldesign (e.g., shown in diagrams 1100 a-1100 c). In some embodiments,this configuration is used for the side-by-side configuration (see,e.g., FIGS. 8A-8D and FIGS. 10A-10D) to reduce pylon width and thusaerodynamic drag at the expense of weight since this way of splittingmotors would lead to weight increases in the overall drivetrain.

Whereas the non-redundant drivetrain (1100 a-1100 c) has a singledrivetrain (1102 a-1102 c), the reduced-height redundant drivetrainexample (1110 a-1110 c) and reduced-diameter redundant drivetrainexample (1120 a-1120 c) both have a first drivetrain (1112 a-1112 c and1122 a-1122 c) as well as a second drivetrain (1114 b-1114 c and 1124b-1124 c). In some embodiments, the two drivetrain systems havecompletely separate electrical systems (e.g., separate batteries) wherethe drivetrains are arranged coaxially, with the motors turning the sameshaft which in turn is connected to the propeller (blades).

With the coaxial arrangement shown here, losses due to belts or gears iseliminated. Another advantage is that since the motors are arrangedalong a common axis, the pylon widths stay the same and the pylonlengths are substantially the same. As a result, the (overall) drag isalmost the same as the original design.

In some applications, redundant drivetrain systems are not deployedacross a vehicle in a uniform and/or homogenous manner. For example, insome embodiments, only the outermost tiltrotors in the exemplary vehicleshown in FIGS. 1A-1B have redundant drivetrain systems, while the other(e.g., inboard) tiltrotors only have a single drivetrain per tiltrotorbecause in case of a failures, it is hardest to compensate for theoutermost rotors due to the long moment arm they have in the rollorientation. Or, in some embodiments, the canard tiltrotors havecoaxially-arranged, reduced-diameter redundant drivetrain systems whilethe other (i.e., main wing) tiltrotors have coaxially-arranged,reduced-height redundant drivetrains systems since it is easier to embeda thicker pylon into the main wing while keeping drag low rather than atthe end of a skinnier canard.

Although the foregoing embodiments have been described in some detailfor purposes of clarity of understanding, the invention is not limitedto the details provided. There are many alternative ways of implementingthe invention. The disclosed embodiments are illustrative and notrestrictive.

What is claimed is:
 1. A system, comprising: an inboard tiltrotorsubsystem, including: an inboard pylon; an inboard tiltrotor; and asingle and non-redundant drivetrain; and an outboard tiltrotorsubsystem, including: an outboard pylon, wherein the outboard pylon iscoupled to a wing; an outboard tiltrotor, wherein: the outboardtiltrotor has a range of motion; and the outboard tiltrotor is coupledto the wing via the outboard pylon, such that the outboard tiltrotor isaft of the wing; a redundant drivetrain, including a plurality of motorsand a plurality of motor controllers, that drives one or more blades;and the one or more blades.
 2. The system recited in claim 1, whereinthe system is included in an electric vertical takeoff and landing(eVTOL) tiltrotor vehicle.
 3. The system recited in claim 1, wherein theoutboard tiltrotor subsystem is an outermost tiltrotor subsystem.
 4. Thesystem recited in claim 1, wherein: the plurality of motors includes afirst motor and a second motor; and the first motor and the second motorare independently powered by a first battery and a second battery,respectively.
 5. The system recited in claim 1, wherein the outboardtiltrotor subsystem further includes a belt-driven shaft.
 6. The systemrecited in claim 1, wherein: the outboard tiltrotor subsystem furtherincludes a belt and a shaft; the plurality of motors includes a firstmotor and a second motor; and the first motor and the second motorrotate about non-coaxial axes of rotation and drive the shaft via thebelt.
 7. The system recited in claim 1, wherein the redundant drivetrainincludes a gear-driven shaft.
 8. The system recited in claim 1, wherein:the outboard tiltrotor subsystem further includes at least one gear anda shaft; the plurality of motors includes a first motor and a secondmotor; and the first motor and the second motor rotate about non-coaxialaxes of rotation and drive the shaft via said at least one gear.
 9. Thesystem recited in claim 1, wherein: the outboard tiltrotor subsystemfurther includes a shaft; the plurality of motors includes a first motorand a second motor; and the first motor, the second motor, and the shaftare coaxial.
 10. A method, comprising: providing an inboard tiltrotorsubsystem, including: an inboard pylon; an inboard tiltrotor; and asingle and non-redundant drivetrain; and providing an outboard tiltrotorsubsystem, including: an outboard pylon, wherein the outboard pylon iscoupled to a wing; an outboard tiltrotor, wherein: the outboardtiltrotor has a range of motion; and the outboard tiltrotor is coupledto the wing via the outboard pylon, such that the outboard tiltrotor isaft of the wing; a redundant drivetrain, including a plurality of motorsand a plurality of motor controllers, that drives one or more blades;and the one or more blades.
 11. The method recited in claim 10, whereinthe inboard tiltrotor subsystem and the outboard tiltrotor subsystem arein an electric vertical takeoff and landing (eVTOL) tiltrotor vehicle.12. The method recited in claim 10, wherein the outboard tiltrotorsubsystem is an outermost tiltrotor subsystem.
 13. The method recited inclaim 10, wherein: the plurality of motors includes a first motor and asecond motor; and the first motor and the second motor are independentlypowered by a first battery and a second battery, respectively.
 14. Themethod recited in claim 10, wherein the outboard tiltrotor subsystemfurther includes a belt-driven shaft.
 15. The method recited in claim10, wherein: the outboard tiltrotor subsystem further includes a beltand a shaft; the plurality of motors includes a first motor and a secondmotor; and the first motor and the second motor rotate about non-coaxialaxes of rotation and drive the shaft via the belt.
 16. The methodrecited in claim 10, wherein the redundant drivetrain includes agear-driven shaft.
 17. The method recited in claim 10, wherein: theoutboard tiltrotor subsystem further includes at least one gear and ashaft; the plurality of motors includes a first motor and a secondmotor; and the first motor and the second motor rotate about non-coaxialaxes of rotation and drive the shaft via said at least one gear.
 18. Themethod recited in claim 10, wherein: the outboard tiltrotor subsystemfurther includes a shaft; the plurality of motors includes a first motorand a second motor; and the first motor, the second motor, and the shaftare coaxial.